52 SCIENCE PROGRESS 



sometimes, along with very large amounts of thorium, an almost 

 entire absence of lead in thorianites and thorites. And in some 

 urano-thorites the lead may be noticed to follow the uranium 

 in approximate proportionality, notwithstanding the presence 

 of large amounts of thorium. 1 This is in favour of the assump- 

 tion that all the lead present is derived from the uranium. The 

 actinium is present in negligibly small amounts. 



On the other hand, there is evidence arising from the atomic 

 weight of lead which seems to involve some other parent than 

 uranium. Mr. Soddy, in the work referred to, points this out. 

 The atomic weight of radium is well known, and uranium in 

 its descent has to change to this element. The loss of mass 

 between radium and uranium-derived lead can be accurately 

 estimated by the number of alpha rays given off. From this 

 we get the atomic weight of uranium-derived lead as closely 206. 

 Now the best determinations of the atomic weight of normal 

 lead assign to this element an atomic weight of closely 207. By 

 a somewhat similar calculation it is deduced that thorium-derived 

 lead would possess the atomic weight of 208. Thus normal 

 lead might be an admixture of uranium- and thorium-derived 

 lead. However, as we have seen, the view that thorium gives 

 rise to stable lead is beset with some difficulties. 



If we are going upon reliable facts and figures, we must, 

 then, assume : (a) That some other element than uranium, and 

 genetically connected with it (probably as parent substance), 

 gives rise, or formerly gave rise, to lead of heavier atomic 

 weight than normal lead. It may be observed respecting 

 this theory that there is some support for the view that a 

 parent substance both to uranium and thorium has existed 

 or possibly exists. The evidence is found in the proportionality 

 frequently observed between the amounts of thorium and 

 uranium in the primary rocks. 2 Or : (b) We may meet the 



1 It seems very difficult at present to suggest an end product for thorium, unless 

 we assume that, by loss of electrons, thorium E, or thorium-lead, reverts to a 

 substance chemically identical with thorium itself. Such a change— whether 

 considered from the point of view of the periodic law or of the radioactive theory — 

 would involve many interesting consequences. It is, of course, quite possible 

 that the nature of the conditions attending the deposition of the uranium ores, 

 many of which are comparatively recent, are responsible for the difficulties 

 observed. The thorium and uranium ores are, again, specially prone to alteration. 



2 Compare results for the thorium content of such rocks (appearing in a 

 paper by the author Cong. Int. de Radiologic et d' 1 Electricity vol. i. 1910, p. 373) 

 and those for the radium content, as collected in Phil. Mag., October 1912, p. 697. 



